1. Field of the Invention
The present invention relates to a signal transmission system and a signal transmission method in the system including a data carrier having a memory for storing therein particular data having an identification code, said data carrier being attached to an object such as a moving object, and a main apparatus for effecting a communication with the data carrier by use of a microwave so as to write data in or read data from the memory of the data carrier.
2. Description of the Prior Art
In a manufacturing process of various kinds of products, a data carrier is attached to a piece of work or a work piece or to a carrying plate (such as a pallet) of a workpiece transported on a belt conveyor. The memory of the data carrier is used to store therein an identification code of an object (a workpiece or a carrying plate) onto which the data carrier is to be attached, a job procedure of each process, a job data, and the like. One or more main apparatuses (control apparatuses) are arranged along the conveyor line. When a moving object passes a range where the main apparatus can conduct communications with the data carrier, the main apparatus writes data in or read data from the memory of the data carrier attached to the object.
As an application example of this kind of system, there has been known a system in which a data carrier is installed on an automobile. There is arranged a control apparatus at a predetermined position to transmit a microwave carrier signal. The data carrier receives the carrier signal by means of an antenna so as to modulate the signal depending on data read from the memory and then emits the obtained signal as a reflection wave from the same antenna. The control apparatus receives the reflection wave so as to demodulate the wave, thereby detecting the data.
As an example of the conventional modulation method in a data carrier, there has been known a modulation method described in the Japanese Patent Publication (Kokoku) No. 60-27077. In this method, the data carrier is provided with a reflective phase modulator, which is used to shift the frequency of the received microwave carrier so as to produce a sideband wave of the carrier such that the sideband wave is modulated by use of data to be transmitted, thereby transmitting the modulated wave as a reflection wave. The control apparatus transmitting the carrier receives only the sideband wave so as to effect a demodulation on the sideband wave to attain the data.
However, in a case where a metal object exists in the proximity of the data carrier, a reflection wave from the metal object overlaps with the reflection wave from the data carrier and hence the signal-to-noise (S/N) ratio is deteriorated, which leads to a problem that a correct data transmission cannot be effected. In addition, in a case where the frequency of the sideband wave is in the neighborhood of the carrier frequency, if the system includes a filter circuit not having a sufficient frequency selecting capability, the signal-to-noise ratio is also deteriorated and hence an error is likely to take place.
On the other hand, there has been employed a system in which a main apparatus sends carriers associated with a horizontally polarized wave and with a vertically polarized wave such that a circular polarized microwave associated with a rotation in a direction is supplied to a data carrier, which then sends as a reflection wave a cicular polarized microwave associated with a rotation in a reverse direction. With this provision, it is possible to discriminate the reflection wave of the metal object from that reflected from the data carrier. However, this method is attended with a problem that the antenna system becomes to be complicated.